1. Field of the invention
The present invention relates to a light sensitive semiconductor device and/or light sensitive semiconductor unit and more particularly to a light sensitive semiconductor device and/or light sensitive semiconductor unit used for supplying electric signal to driver semiconductor elements such as light sensitive semiconductor switches or light sensitive semiconductor relays.
2. Description of the Prior Art
It has been well known to use a light sensible semiconductor switch employing one or more MOS field effect transistor (referred to as FET). In the conventional light sensitive semiconductor switch, there is employed a light sensitive semiconductor cell which generates an electric power by receiving light emitted from a light source such as a light emitting diode (referred to as LED). As the light sensitive semiconductor cell, poly silicon semiconductor, single crystal silicon semiconductor or amorphous semiconductor are used. The following explanation is made with reference to the light sensitive semiconductor unit made of amorphous semiconductor material.
It is noted that in the disclosure, the term "light sensitive semiconductor cell" generally means an assembly of a light sensitive semiconductor element with a pair of connecting electrodes, the term "light sensitive semiconductor unit" generally means an assembly of a number of light sensitive semiconductor cells formed on a substrate and the term "light sensitive semiconductor device" means an assembly of the light sensitive semiconductor unit and other elements such as LED or the like.
Referring to FIGS. 1a to 1c, twenty four lower electrodes 21 each made of a transparent electrically conductive film such as indium tin oxide (ITO) or tin oxide (Sn O.sub.2) are disposed in a matrix shape on a top surface 20a of an insulation substrate 20 having a rectangular shape of about 1 to 5 mm wide. Each of lower electrodes 21 is electrically insulated. A light sensitive semiconductor element 22 is formed on the upper surface of every lower electrodes 21. On the upper surface of each semiconductor element 22, an upper electrode 23 made of metal such as aluminum is formed. By the arrangement mentioned above, there are formed 24 light sensitive semiconductor cells on the substrate 20. Each of the light sensitive semiconductor cells has an equal light receiving area. Moreover, each of the light sensitive semiconductor cells has its projecting . portion of the upper electrode electrically connected to the lower electrode of the adjacent cell so that 24 light sensitive semiconductor cells can be electrically serially connected. The respective light sensitive semiconductor cells are coated with a protecting film made of electrical insulation material.
The light sensitive semiconductor unit 30 in which 24 light sensitive semiconductor cells are arranged as mentioned above is disposed in such a manner that the center C of the unit 30 opposed to a light source 7 or 8 (see FIGS. 2a or 2b) with a separation about 0.5 mm to several millimeters. The light from the light source 6 is illuminated on the surface of the substrate 20 and, the light reaches the respective semiconductor elements 22 passing through the lower electrodes 21. Upon receiving the light, the light sensitive semiconductor unit 30 generates electrical power and the generated electrical powers are serially added and taken out from the terminals 24 and 25.
Since the rays of light from the light source such as LED are not parallel, the light intensity received by the respective light sensitive semiconductor cells is not uniform. In general, in the case where LED is disposed so it faces the center of the light sensitive semiconductor unit 30, the light intensity is strongest at the center C of the unit 30 and the light intensity becomes lower as the distance from the center C increases. Therefore, in the conventional light sensitive semiconductor device as shown in FIGS. 1a to 1c, since the light receiving area of the respective light sensitive semiconductor cells is substantially equal, the electric power generated at the central cell 26 and that generated at the cells 27 situated at the respective corners are different depending on the intensity of the light incident to the respective light sensitive semiconductor cells. That is, if the light intensity received by the cell 27 is a half of the light received by the cell 26, the power generated at the cell 27 is a half of the power generated at the cell 26. In the case where the light sensitive semiconductor device is formed by connecting a number of the light sensitive semiconductor cells in series, nonuniform light intensity as mentioned above causes a problem in the light sensitive semiconductor device. Referring to FIG. 3 showing a general characteristics of the light sensitive semiconductor unit 30, wherein Isc is a short circuit current of the light sensitive semiconductor unit and Voc is an open circuit voltage of the light sensitive semiconductor unit. The short circuit current Isc is generally proportional to the light intensity of the light source in the case where the light receiving area of the light sensitive semiconductor cell is the same at any position on the cell. On the other hand, the open circuit voltage Voc scarcely changes irrespective of change of the light intensity of the light source. Substantially similar characteristics can be obtained for the light sensitive semiconductor unit in which a number of the light sensitive semiconductor cells are serially connected. However, the short circuit current Isc is defined by the light receiving area of the single light sensitive semiconductor cell and the open voltage is defined by the open voltage Voc.times.number of the stages of the cells. In the conventional light sensitive semiconductor unit having semiconductor cells of the same light receiving areas, there can be obtained such a characteristic as shown in the dotted line in FIG. 4. In the case where the light intensity decreases as the distance from the center of the light sensitive semiconductor unit increases, the short circuit current of the light sensitive semiconductor unit 30 is defined by the short circuit current Isc2 of the semiconductor cell 27 situated at the position farthest from the center even if the short circuit current Isc1 of the central semiconductor cell 26 is greater than the short circuit current Isc2 and the characteristic curve of the light sensitive semiconductor unit is shown as the real line in FIG. 4. Under such a condition, when a load R1 is connected to the light sensitive semiconductor unit 30, the operating voltage OP2 appearing across the load is lower than the operating voltage OP1 appearing across the load when the light intensity incident to the respective light sensitive semiconductor cells is uniform.